Mendelelian Genetics

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Mendelelian Genetics
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Generation Gap

• P1, patrial generation the parents or first two
  organisims crossed.
• F1, first filial generation the first set of
  offspring
• F2, second filial generation the result of two
  of the F1 generation being crossed.
• Developed the terms gene,allele,
  homozygous,heterozygous, dominent, recessive,
  genotype and phenotype.

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Mendels Laws1. Law of Dominance2. Law of
Segregation3. Law of Independent assortment
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• Law of Segregation

• Each paired gene must seperate during gamete
  formation so alleles cqn recombine in new pairs.

Law of Independent Assortment
Law of Independent Assortment
Law of Independent Assortment

• Traits are inherited independantly and are not
  changed by other alleles for other traits.

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Results of Monohybrid Crosses

• Inheritable factors or genes are responsible for
  all heritable characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one from the
  mother and the other from the father
• True-breeding individuals are homozygous ( both
  alleles) are the same

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• Back or test cross to test for homo or
  heterozygous.

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Incomplete DominanceandCodominance
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Incomplete Dominance

• F1 hybrids have an appearance somewhat
  in between the phenotypes of the two parental
  varieties. Neither gene is dominent both are
  expressed equally.
• RR red flower
• rr white flower
• But Rr makes pink flowers!

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Incomplete dominance problems

• In Andalusian chickens, the black Andalusian
  character is incompletely dominant to the
  white-splashed Andalusian character. The
  heterozygous condition produces blue Andalusian
  chickens. Determine the genotypes and phenotypes
  of the F1 and F2 generations if a pure breeding,
  black Andalusian is crossed with a pure breeding,
  white-splashed Andalusian.

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Codominance
Both alleles are equally expressed. The offspring
have a mix of alleles which are equally
expresses. Eg. Red cows crossed with white will
generate Roan cows. But F2 generation
demonstrate Mendalian genetics (121)
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Codominance vs Incomplete

• In incomplete dominance only one allele is active
  but is reduced in effect.
• In codominence both alleles are active. In human
  blood this produces a whole new blood group - AB.
• 1. type A IAIA or IAi
• 2. type B IBIB or IBi
• 3. type AB IAIB
• 4. type O ii

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• How we write it - CrCw

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OVERDOMINANCE

• The phenotype falls outside the range of the
  parents e.g if one homozygote is tall (TT) and
  the other is short (tt) then the heterozygote may
  be extra tall (Tt). If this is a good adpation
  then it can become common in the population.

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LETHAL GENE

• A mutation of a gene that produces a product that
  is nonfunctional. In some the homozygous dominant
  is lethal it dies as an embryo so get a ratio
  of 21 instead of 121. In some it just affects
  expression of genes (Manx cats) and it can also
  be expressed at different stages of development
  e.g. Huntingtons.

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Manx Cat

• Manx Cat Tailless cat is another trait caused
  by an allele that has dominant effect in
  heterozygous and is lethal in homozygotes.
• The Manx and normal alleles are denoted by L and
  l respectively
• MLMl or MLML

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• Other example is achondroplasia, the most common
  form of dwarfism, with a normal length body trunk
  but shortened limbs. AA die, aa normal, Aa -
  dwarf.

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Multiple Alleles

• More than one gene at a locus coding for a trait.
  Human blood type is a great example there are
  three alleles for blood type, A,B and O. Allele
  A and B are co-dominant and both are dominant
  over O.
• Reminder the truth about eye colour.

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Try These

• 1. If a male has blood type B and a female has
  blood type A, what are the possible blood types
  in the offspring?
• 2. Is it possible for a child with Type O blood
  to be born to a mother who is type AB? Why or
  why not?
• 3. A child is type AB. His biological mother is
  also type AB. What are the possible phenotypes
  of his biological father?

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Gene Interaction and Epistasis

• The phenomenon of two or more genes governing the
  development of single character is known as Gene
  interaction. The genes can be on different
  chromosomes.
• When one gene masks or alters the expression of
  another gene, the phenomenon is called Epistasis.
  (eg and meaning)

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3.Supplementary Gene Action

• This is the masking of a characteristic (which is
  determined by one pair of alleles) by the action
  of another pair of alleles.
• An example is mouse fur one allele gives black
  (B) lots of melanin, when recessive (b) it
  gives brown less melanin. Another gene allows
  how much melanin is deposited so C allows colour
  to show but c does not.

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Collaboration

• This is where one characteristic is controlled by
  two or more pairs of alleles. The two genes
  interact to produce a novel phenotype. One well
  known example is comb shape in chickens. The
  four shapes are single,pea,walnut,rose.
• The ratio is the same as a normal dihybrid cross
  9331
• P_R_ - two dominants gives walnut
• P_rr one dominant one recessive gives pea
• ppR_ - other dominant and other recessive gives
  rose
• pprr two recessives gives single.
• The big difference to a normal dihybrid cross is
  the fact that the phenotypes are four differnt,
  not variations of the same two traits.

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• The ratio is 943.
• B_C_ - black
• B_cc white
• bbC_ - brown
• bbcc - white

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Types of Epistasis

• 1. Collaboration
• 2. Complementary
• 3. Supplementary

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2. Complementary gene action

• In this case two dominant allelees from two
  different genes are needed to get expression of
  the gene. The genes are complementary to each
  other. The ratio is 97.
• As an example sweet peas the development of
  purple flowers requires the presence of 2
  dominant genes, C and R, e.g., CCRR.
• When either C or R alone present purple flowers
  cannot be produced as a result white flowers are
  obtained
• e.g., ccRR or CCrr or ccrr

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Pea
Rose
Single
Walnut
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• Linkage and sex determination

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What is so different between the X and Y
chromosomes?

X- over 1000 genes identified Y- 330 genes
identified, many are inactive One gene on the Y
is very important SRY. The SRY gene is the
primary determinant of sex. If SRY is present,
testes develop in the early embryo. The testes
secrete the hormone testosterone, which causes
development as a male. If SRY is absent (no Y
chromosome), ovaries develop instead of testes,
and the embryo develops into a female.
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The sex chromosomes

• Many organisims like humans with a homogamteic
  sex (female XX) and a heterogametic sex (males
  XY).
• XY- female, XX- Male This system of sex
  determination operates in birds, reptiles, some
  insectsThe bird sex chromosomes are called Z and
  W.

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• In some insects there is no Y chromosome so you
  get XO and XX.
• Diploid (2n) female, Haploid (n) maleThis system
  of sex determination is found mainly in
  Hymenoptera honey bees, ants, termites, etc.
• The unfertilized haploids develop male and called
  drones.
• These drones carry only half the number (16) of
  chromosomes of the female (32)

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• In reptiles and some fish sex is determined by
  the environment ( temperature, abilty to change
  sex etc).

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Family trees

• 3 types sex-linked
• autosomal recessive and autosomal dominant

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Sex Linked Inheritance

• Because the X chromosome is larger than the Y
  there are parts of the X chromosome that have no
  matching part on the Y. Any gene carried on the
  non-homologous part are called sex linked.
  Examples are red-green colour blindendss,
  haemophilia, all tortiseshell cats are female.
• For males, any faulty gene on the X will show up
  as there is no gene on the Y to mask the effect.
  In females both parents must have the recessive
  trait to pass it on.
• We write the alleles above the X and Y symbol.

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• Sex-linked notation
• XBXB normal female XBXb carrier female
• XbXb affected female XBY normal male
• XbY affected male
• SEX LINKED DOMINANCE
• Dominant gene on X chromosome
• Affected males pass to all daughters and none of
  their sons
• Genotype XAY

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• If the mother has an X- linked dominant trait and
  is homozygous (XAXA) all children will be
  affected
• If Mother heterozygous (XAXa) 50 chance of each
  child being affected
• E.g. dwarfism, rickets, brown teeth enamel
• SEX LINKED RECESSIVE
• Gene located on the X chromosome
• More males than females affected (males inherit X
  from mother)
• Females can only inherit if the father is

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Sex linked dominant disorders
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• affected and mother is a carrier (hetero) or
  affected (homo)
• An affected female will pass the trait to all her
  sons
• Daughters will be carriers if father is not
  affected
• Males cannot be carriers (only have 1 X so either
  affected or not)
• E.g. colour blindness, haemophilia, Duchene
  muscular dystrophy

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• 2. Duchene type Muscular Dystrophy
• Is also depends on sex linked recessive gene.
• If mother is carrier, about half of the male
  children are expected to be affected.
• Can be identified by chromosome study.
• It affects male before they reach teens, with
  muscular deterioration .
• Muscles of leg and shoulders become stiff and the
  children usually become paralyzed and crippled
  during their middle or late teens.
• Virtually all die before age of 21.

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Sex-influenced Genes

• Some traits even though not on sex
  chromosomesappear differnntly in men and women
  e.g. pattern baldness. It is an inherited trait
  and controlled by one gene. In females it acts
  as a recessive so a woman needs two recessives to
  show baldness. In men only one is needed BB
  full hair both sexes, Bb bald in men not in
  women, bb bald in both.

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• The Adams family and Baldness

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Colour blindness

• We have 3 color receptors in the retinas of our
  eyes. They respond best to red, green, and blue
  light.
• Each receptor is controlled by a gene. The blue
  receptor is on an autosome, while the red and
  green receptors are on the X chromosome
  (sex-linked).

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Features of Colour blindness

• Colour blindness is a sex linked character
  discovered by Wilson
• It is a hereditary disease and the affected
  person cannot distinguish green and red colour.
• The red blindness is called Protonopia, these
  persons cannot see red colour
• While, green blindness is called deuteronopia,
  such a persons cannot see green colour.
• Colour blindness is a recessive character,
  represented by cc
• The genes are for colour blindness is located on
  X chromosome. It is common in male but rare
  female.
• Colour blindness follows criss- cross inheritance
  as transmitted from father to grandson through
  daughter.
• It is never transmitted from father to son

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Linkage

• It is universally accepted that genes are located
  in chromosomes
• Linkage The tendency of two or more genes to
  stay together during inheritance because they are
  located on same chromosome.

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Recombination During Meiosis
Recombinant gametes
Parental gametes
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Complete and Incomplete linkage

• Gene show a linkage because they are located on a
  chromosome.
• For e.g., C and Sh are present in one chromosome,
  while their recessive alleles c and sh are
  situated in the homologous chromosome.

C sh
c Sh
c Sh
C sh
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• Each chromosome behaves as a unit during cell
  division.
• Therefore, C and Sh would move to one pole to
  while c and sh move to opposite pool.
• If this always happened, F1 generation (CcShsh)
  would produce 2 gametes viz., C Sh and c sh
• When only parental character combinations are
  recovered in test progeny , it is called complete
  linkage.
• However, sometime, allele recombine to produce
  recombinant types like C sh, cSh
• Such a type are called Incomplete linkage.

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• Linkage is never 100. No matter how tightly two
  genes are linked, if you observe enough
  individuals, you will find some recombinants.

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Linkage Mapping

• Each gene is found at a fixed position on a
  particular chromosome. Making a map of their
  locations allows us to identify and study them
  better.
• In modern times, we can use the locations to
  clone the genes so we can better understand what
  they do and why they cause genetic diseases when
  mutated.
• Thus, the percentage of gametes that had a
  crossover between two genes is a measure of how
  far apart those two genes are.

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• Pleiotrophy one gene with many different
  effects. Often seen in genetic diseases such as
  sickle cell aneamia.

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Sickle Cell Anemia
Under conditions of low oxygen tension,
hemoglobin S will precipitate, causing cells to
sickle Mutations in one amino acid HH
normal and gets maleria Hh protected from
maleria but gets sickle cells in low O2 Hh dies
young from anaemia.
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• Sickle cell anemia may be the result of a genetic
  mutation that happened in malaria-prone regions
  like Africa thousands of years ago.
• People with sickle cell trait may have been more
  likely to survive malaria epidemics - and
  because they survived when others did not, this
  allowed the trait to be passed down through
  generations.

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• Polygenes two or more genes have similar and
  additive effects on the same characteristic.
  E.g. intelligence, height, skin colour. The
  phenotypes for these genes form a bell shaped
  curve and show continuous variation.
• e.gs

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Genes and Environment Determine Characteristics
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Gene/Environment Interactions

• Genes determine range of genotypes but
  environmental factors fine tune which phenotype
  is displayed.
• Internal and External Environment affect
• Internal
• modifier genes sometimes the expression of the
  gene at one locus is affected by alleles at
  another locus.
• Sex-limited genes
• Sexlinked genes sex hormaones affect.

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• External environment
• Drugs
• Startvation or malnutrition
• Lack of light
• Temperature
• Ionising radiation
• Poluuting chemicals